Sensors for detecting and measuring absolute or relative values of physical quantities such as chemical or biochemical concentration, magnetic or electric field strengths, pressure, strain, temperature, and pH, for example, in an environment to which the sensor is exposed, are well known in the art. Prior art sensors include direct reading sensors, which include, for example, a mercury thermometer or Bourdon pressure gauge, and sensors that employ transducers for converting an input signal or stimulus into an output signal of a different type. Thus, an infrared pyrometer converts infrared radiation into a useful electrical output signal readable by an electrical meter.
Prior art sensors also include optic sensors which provide measured values directly or by means of transducers. A simple color comparison pH test apparatus is an example of optic sensor read directly whereas a camera light intensity metering system is optic sensor employing a transducer. The most sensitive optic sensor is of the interferometric type, which employs an interferometer to provide information about a condition sensed. An interferometer is an instrument that splits light from an input source into two or more light beams. The light beams are caused to travel through different paths with different effective optical path lengths so that an interference fringe pattern is produced when the beams are recombined. An analysis of the light and dark bands of the interference pattern provides a sensitive measure of the difference in effective path length of the different optical paths.
A particular group of optic sensors which has experienced significant technical development in recent years includes integrated optic sensors. Integrated optic sensors are monolithic structures characterized by the integration of various optical components into a single optic waveguide construction. An integrated optic sensor is typically a thin-film device comprising a waveguide constructed on a single substrate, which generally provides other optical elements or components to diffract, refract, reflect, or combine different beam portions propagating in the waveguide. Integrated optic technology is particularly useful in providing the optical elements heretofore associated with interferometric sensors employing separate and discrete optical components. The prior art now includes integrated optic sensors that incorporate a variety of components including lenses, sensing fields, and filters on a single substrate.
A typical integrated optic sensor comprises one or more channel waveguides fabricated as a planar construct on a substrate. A channel waveguide is a linear structure of typically small cross-section, on the order of several micrometers wide by several micrometers high, providing an optical path for a propagating light beam. The index of refraction of the channel waveguide is higher than the index of refraction of the surrounding or supporting substrate. A light source and possibly a coupling mechanism are provided to cause a light beam to propagate within the channel waveguide. The light source can be a laser, a light emitting diode (LED), or an incandescent light source. The propagating light beam passes through a sensing region of the channel waveguide which is reactive to particular conditions of the environment. The environment may cause changes in the propagation characteristics of the channel waveguide, such as a change in the refractive index. The change in the refractive index changes the effective path length through the channel region, thereby changing the phase of the light beam as it emerges from the channel waveguide. Alternatively, if the channel waveguide is not directly sensitive to a particular environment, it may be coated with a material that is reactive to the environment, or to a component thereof, causing a change in the refractive index of the channel waveguide. An optical output beam from the sensor can therefore be used for measuring the relative or absolute value of the condition of the environment.
The optical input beam propagates through the waveguide in modes which satisfy the well-known Maxwell equations. Maxwell equations govern the electric and magnetic fields of an electromagnetic wave propagating through a medium. The modes may be characterized by the frequency, polarization, transverse field distribution and phase velocity of the constituent waves. In rectangular channel waveguides the modes are designated as TE.sub.m,n and TM.sub.m,n which are orthogonally polarized components of the light beam, transverse electric and transverse magnetic, respectively, with mode number indices m and n taking non-negative integer values. Each mode represents a different field distribution corresponding to the number of wave nodes across the waveguide in each direction. The allowed modes are determined by the configuration of the boundaries of the waveguide, which for integrated optic sensors are the interfaces between the substrate and waveguide, the environment and the waveguide, and/or the coating and the waveguide. Depending on the boundaries and the wavelength of the input light source, no modes, one mode, or more than one mode may be allowed to propagate through the waveguide.
Commercially available integrated optic interferometers include those utilizing a Mach-Zehnder interferometric technique, such as that disclosed in U.S. Pat. No. 4,515,430, which is incorporated herein by reference. This technique is characterized by single mode propagation of two light beams through two light paths, then combining the two beams to produce an optical interference pattern. Generally, a Mach-Zehnder device receives a single input light beam which is then split by a beam splitter into two beams that are directed through two different channel waveguides. Changes in the optical path length of one of the waveguides are effected when the environment causes a change in its physical length or a change in its refractive index. The beams emerging from the channel waveguides are recombined to produce a single interfering beam which is indicative of the relative or absolute change caused by exposing the device to the environment.
Integrated optic interferometers employing the Mach-Zehnder configuration provide outstanding sensitivity and can be made in small sizes. These sensors, however, suffer in that they rely on two or more single-mode channel waveguides with typical cross-sectional dimensions of 2 .mu.m by 3 .mu.m each, making fabrication difficult and costly. Furthermore, the two channel waveguides can be affected differently by thermal and vibration aspects of the environment, leading to spurious interference effects. Most importantly, the small size of the channels make efficient light coupling difficult to achieve with Mach-Zehnder interferometers. The light coupling difficulty makes this type of interferometer all but useless for many applications.
A second type of integrated optic interferometric sensor uses a planar waveguide as the planar construct. A planar waveguide is defined by only two (parallel) boundaries, rather than the four rectangular boundaries typical of a channel waveguide. In a planar waveguide, the propagating modes are designated as TE.sub.m and TM.sub.m (transverse electric and transverse magnetic, respectively), with the mode number index m taking non-negative integer values. As in the channel waveguide, the boundaries and the wavelength of the input light source determine whether no modes, one mode, or more than one mode may be allowed to propagate through the waveguide.
In the descriptions of the prior art and of the invention in this specification, the terms "planar construct" and "planar waveguide" refer to structures that are generally planar in their overall construction. However, it should be understood that planar constructs and planar waveguides can incorporate such features as grooved or ridged surfaces, porous layers or regions, or other embedded or surface relief features. In addition, the descriptions herein of the prior art and of the invention use the term "optic" and "light," but it must be recognized that the techniques described are phenomena of electromagnetic radiation in general. Thus, the term "optic" and "light" herein should be read as referring to any electromagnetic radiation that meets whatever constraints are imposed by the characteristics of the various components of the sensor (such as the dimensions of the optical path) and the nature of the interaction between the sensor and properties of the environment to be sensed (such as the sensitivity of the sensor as a function of wavelength). Typically, the light will be in the visible or near-visible wavelength range.
Planar waveguide interferometric sensors are disclosed in U.S. Pat. No. 4,940,328, issued to Hartman, and U.S. Pat. No. 5,120,131, issued to Lukosz, which patents are incorporated herein by reference. In the devices described in these patents, light is propagated in at least two modes in a single planar waveguide, as opposed to propagating in multiple channel waveguides, and passes through a sensing field exposed to the environment to be measured. The sensor output includes a beam comprising the interference products of at least two of the modes propagating in the planar waveguide. The higher order modes are most influenced when the environment causes a change in the refractive index of the planar waveguide and, when combined with the lower order modes, provide an output showing intermodal interference. These devices are more easily fabricated than the single mode Mach-Zehnder devices and are characterized by easier light coupling into the waveguide. However, the sensitivity of these devices is generally only one-half to one-third the sensitivity of the Mach-Zehnder devices. The lower sensitivity is the result of the fact that the lower order mode is not completely unaffected by the environmental stimulus and it therefore does not provide a true reference beam.
Another problem with the prior art devices is their inability to achieve high levels of integration of optical components in a single waveguide utilizing a single processing technology.
The above deficiencies greatly reduce the usefulness of integrated optic sensors and increase their cost. What is needed and what is not currently available is an integrated optic sensor that is relatively inexpensive, incorporates a high degree of component integration into a single waveguide, and provides high sensitivity with simple and efficient light coupling.